The equation of a circle $C$ is $(x-2)^{2}+(y+4)^{2} = 36$. What are its center $(h, k)$ and its radius $r$ ?
Answer: The equation of a circle with center $(h, k)$ and radius $r$ is $(x - h)^2 + (y - k)^2 = r^2$ We can rewrite the given equation as $(x - 2)^2 + (y - (-4))^2 = 6^2$ Thus, $(h, k) = (2, -4)$ and $r = 6$.